In general, power generation equipment operates most efficiently under steady-state conditions that allow design engineers to optimize operating conditions. To the extent that power generation equipment operates outside these optimum design conditions, energy efficiency generally declines. For example, if a power plant must be turned down to produce a lower amount of power, then flue gas energy recovery systems may become less efficient, resulting in an overall decrease in conversion of energy to useful electrical energy. On the other hand, when a power generating plant operates at above its designed capacity, inefficiencies may result from factors such as incomplete combustion and the inability of energy recovery systems to recover a high proportion of incremental heat produced.
Although energy-efficient operation of power generation equipment requires a steady-state load, demand for power varies cyclically throughout a day, and also varies seasonally. Thus, it is not often feasible to operate power plants at optimum levels of efficiency, since exactly corresponding demand does not occur for any great length of time. Energy generation systems are also subject to faults, which cause voltage and phase changes and outages, which are not acceptable to many users. These users must have an assurance of power quality and/or an auxiliary energy storage or generation system to prevent outages.
Efforts have been made to "smooth" the demand for energy from power plants to facilitate steady-state power generation equipment operation. Some of these efforts have focused on auxiliary power generation equipment that may be operated when demand is high and shut down, or "turned down", when demand is low. These auxiliary units are not only expensive but are usually also inefficient, since they also do not operate at their optimum load levels but at varying levels, depending upon demand. Other efforts have focused on energy storage. Examples of such energy storage systems include, for example, the use of batteries to store electricity or the use of pumped storage systems. The pumped systems utilize excess power generated during low power demand periods to pump water to an elevated storage position, thereby imparting potential energy to the water. When demand for energy increases, the water is released from storage and flows to a lower elevation, releasing potential energy, which is typically converted, via turbines, to kinetic energy and subsequently to electricity.
It has also been proposed that excess energy could be stored in large flywheels that are caused to rotate at very high speeds, thereby storing energy as kinetic energy. However, these flywheel energy storage concepts present several challenging issues. A flywheel that rotates on a mechanical bearing will generally suffer relatively high energy losses due to bearing friction. Thus, the ratio of output energy from the flywheel to input energy (a measure of overall efficiency) is often relatively low so that such systems are usually commercially unattractive.
Magnetic bearings have been proposed for a variety of flywheel designs. However, these bearings also suffer significant drawbacks. Permanent or electromagnets do not provide lateral stabilizing forces to hold a rotating flywheel in position. Thus, electromagnets with complex and low efficiency servosystems are required for lateral stability. Also, magnets in a motor/generator are often arranged so that, when the wheel rotates at high speed and components undergo radial expansion, the gap between the magnets and the field coils increases, thereby decreasing efficiency--an undesirable effect.
Also, a flywheel rotating at high speed generates high radial and hoop stresses in the wheel structure. And, the higher the rate of rotation, the greater these forces become. At some point, hoop stresses, which exceed radial stresses, may cause a failure of wheel materials with potentially devastating results. To avoid this eventuality, expensive high strength materials must be used. This high cost discourages the use of flywheels, since it is desirable to use a lowest cost method of energy storage.
A flywheel that is used to store energy may be expected to rotate within a normal operating range of frequencies or rotational speeds related to the highest and lowest amounts of energy stored. Generally, it is undesirable that the flywheel have a critical frequency, which sets up a resonance condition, within this operating range of speeds. However, materials and mechanical designs frequently make it difficult, if not impossible, to entirely eliminate critical frequencies within the normal operating range of the flywheel. This impairs the operating flexibility of the flywheel since it is undesirable to operate through the critical frequency as a normal condition of use.
In order for flywheels to become commercially attractive for use as energy storage devices, the flywheels must be relatively inexpensive to produce, able to store a commercially useful amount of energy, without risk of self-destruction due to radial or hoop stresses, must have a critical frequency that is outside the range of operating conditions, and should have efficient bearings and motor/generators to minimize energy losses.